minor #13244 [Validator] Fix title in sequence_provider.rst (ogizanagi)

This PR was merged into the master branch.

Discussion
----------

[Validator] Fix title in sequence_provider.rst

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

b1d5404 [Validator] Fix title in sequence_provider.rst

Author: javiereguiluz

validation/sequence_provider.rst

@@ -357,7 +357,7 @@ provides a sequence of groups to be validated:
         }
 
 How to Sequentially Apply Constraints on a Single Property
-==========================================================
+----------------------------------------------------------
 
 Sometimes, you may want to apply constraints sequentially on a single
 property. The :doc:`Sequentially constraint</reference/constraints/Sequentially>`